In the metallurgy of steel, injection technique is playing an increasingly important role for the development of continuously improved steel qualities. By injecting fine particle size calcium and/or magnesium compounds or alloys it has thus for example been possible to reduce the total content of sulphur in steel melts to extremely low values. By selecting suitable injection products it has also been possible to transform residual sulphides so that these become as harmless as possible. For example U.S. Pat. No. 4,261,735 describes a method in accordance with which lime, CaO, is initially injected and then when the majority of the sulphur has been removed, injection is continued with calcium-silicon, SiCa, calcium carbide, CaC.sub.2, and/or calcium cyanamide, CaCN.sub.2.
In modern metallurgy it has also been possible to reduce the content of oxidic inclusions to very low levels. Traditionally the majority of the oxygen in steel melts is removed by de-oxidation using a suitable de-oxidising agent, usually aluminium, silicon or manganese. To reduce the oxygen content to values lower than those which can be achieved using aluminium, silicon and manganese it is necessary however to employ more effective oxidising agents. The substances which can be mainly considered are earth alkali metals in the elementary or non-oxidic-combined form. In actual practise this usually signifies calcium, although also magnesium can be employed as an alternative to calcium. For example calcium can be added in metallic form, as a wire encapsulated in a plate metal casing. Calcium can also be added as calcium-silicon, although the maximum permissible silicon content in the steel imposes a limit on how much calcium-silicon can be added. Furthermore in actual practice it has proved difficult to reduce the oxygen content in this way. The same applies to the injection of calcium carbide CaC.sub.2, which in addition signifies that the melt is carburised and this is not acceptable in many cases, particularly not when producing low carbon content stainless steel or other steels with very low carbon content.
In the search for increasingly improved grades of steel requirements have now been imposed for extremely low oxygen contents, meaning not more than 5 ppm and preferably not more than 2 ppm dissolved oxygen in the liquid steel after the injection treatment. The aim of these extremely low oxygen contents is to reduce the total quantity of oxidic inclusions in the finished steel. By reducing the content of oxidic inclusions, mainly inclusions of the Al.sub.2 O.sub.3 type, it is thus possible to considerably reduce the number of rupture initiators. Furthermore the polishability of the steel can be improved which in many cases is a desireable property for stainless steel and tool steels. The low oxygen content also has advantages from the viewpoint of the matallurgical process, because a low content of oxidic inclusions improves the viscosity of the steel and reduces the risk of blockages during the casting of the steel. Furthermore a low oxygen content is a pre-requisite if extremely low sulphur contents are also to be achieved, because the oxygen activity in the steel melt determines the course of desulphurisation.
The extremely low oxygen contents required in accordance with the invention, i.e. not more than 5 ppm and preferably not more than 2 ppm, could not however be achieved at reasonable cost with high reproducibility using methods known hitherto unless at the same time quality requirements other than the low oxygen content were abandoned.